Lateral motion of beta receptors in membranes of cultured liver cells.

We have studied the lateral mobility and distribution of beta receptors on Chang human liver cells by fluorescence photobleaching recovery and video intensification microscopy. The beta receptors were labeled with the fluorescent antagonist 7-(2-allylphenoxy)-2,2-dimethyl-6-hydroxy-1-(4-nitrobenzo-2-oxa-1,3-diazolyl)-1 ,4-diazaheptane (Alp-NBD). Sixty to 75% of the staining was specific (displaceable by unlabeled antagonists). Most of the antagonist-occupied beta receptors were immobile, because only 15-25% of their fluorescence recovered on the experimental time scale at 23 degrees C. This immobility correlates with the clustered distribution of Alp-NBD--beta-receptor complexes at 4 degrees C and 37 degrees C. The beta receptors appear to be aggregated prior to antagonist binding, because visible patches were observed immediately after labeling for 30 sec at 4 degrees C. Preincubation at 37 degrees C with (--)-isoproterenol, a beta agonist, prior to Alp-NBD labeling induced a time-dependent release of the beta receptors to a more homogeneous distribution and increased the mobile fraction to 70-80% (lateral diffusion coefficient = 1.4 X 10(-9) cm2/sec at 23 degrees C). This is not due to an effect on membrane fluidity, because the diffusion coefficient of a lipid probe was not altered. The time course of agonist-induced beta-receptor mobilization correlates with receptor loss and adenylate cyclase desensitization but is much slower than adenylate cyclase activation. This indicates that adenylate cyclase activation by beta receptors does not require macroscopic lateral mobility of the majority of the beta receptors.